Structure and reactivity of copper-zinc-cadmium chromite catalysts

Abstract

Different mixed oxides containing Cu, Zn, Cd and Cr were obtained by heating hydroxycarbonate precursors at various temperatures and in different atmospheres, and characterized by using X-ray diffraction (XRD), infrared, diffuse reflectance spectroscopy, thermogravimetry, and BET and copper surface area analysis. The reduction process and the phase evolution during reduction of the mixed oxides was followed by temperature-programmed reduction and XRD. The mixed oxides were activated in an H 2/N 2 flow and then tested as catalysts in the vapour-phase hydrogenation of γ-butyrolactone (GBL) and a solution of maleic anhydride (MA) in GBL (60:40 w/w). Depending on composition, heating temperature and atmosphere, the samples contained different phases, such as CuO, CdO, Cr 2O 3, CuCrO 2, CuCrO 4, α-CdCrO 4, cubic and tetragonal CuCr 2O 4, and cubic Cu Zn and Cu Cd spinel-type phases. The partial substitution of Cu 2+ ions with Zn 2+ or Cd 2+ ions stabilizes the cubic form of the spinel-type phases, which form via intermediate chromate phases. The final product of reduction in all cases is metallic copper. Cubic CuCr 2O 4 is more reducible than the corresponding tetragonal phase, which reduces to metallic copper through the intermediate formation of CuCrO 2. The presence of zinc or cadmium promotes or inhibits, respectively, the copper reducibility both in CuO and in the copper chromite spinel. In the hydrogenation of GBL, the Cu/Cr catalysts obtained by calcination show similar behaviours, with a small increase in activity as a function of the copper content. At the lower temperatures investigated the main products are tetrahydrofuran (THF) and n-butanol, while at 548 K significant amounts of ethanol are observed, favoured by increasing copper content. On the other hand, the Cu/Cr catalyst obtained by heating under a reduced oxygen atmosphere, for which only tetragonal CuCr 2O 4 is detected before reduction, shows a decrease in catalytic activity. Partial substitution of the Cu 2+ ions gives rise to a decrease in GBL conversion, with a considerable deactivation when Cd 2+ ions are present. Decreasing the H 2/C 4 molar ratio gives rise to a decrease in activity for all samples, without any significant change in the trend observed. With the MA/GBL solution, the partial substitution of Cd 2+ ions for Cu 2+ ions also gives rise to considerable deactivation. For the other catalysts, the main products at the lower temperatures are GBL and succinic anhydride (SA), while at 548 K overhydrogenation and hydrogenolysis reactions predominate. The presence of Zn 2+ ions gives rise to an increase in yield in GBL, due to an increase in the hydrogenation activity toward SA, but the consecutive transformation of GBL at high temperature to low cost by-products is not inhibited. On the other hand, the presence in the Cu/Cr catalyst of only CuCr 2O 4 increases the yield in GBL, as a consequence of a strong inhibition of the overhydrogenation and/or hydrogenolysis reactions of GBL. With the MA/GBL solution, similar behaviours are observed when the H 2/C 4 molar ratio decreases. At the same time, however, the lack in the carbon balance increases and a displacement towards high temperature of the maximum yield in GBL is found.